A prior art metal mold for plastic molding a semeiconductor device used in a transfer-molding method constitutes a cull portion which, provided at the center of the mold, forms a base as a starting point of the plastic molding to which the plastic is introduced from a transfer pot by an injecting cylinder. The mold also includes a runner section and a gate section which form a path from the cull section to a cavity section where a product is completed.
FIG. 5 shows a prior art metal mold for plastic molding a semiconductor device in which a cull section, a runner section, and a gate section are provided at a lower mold. In the drawing the reference numeral 1 designates a cull section, the numeral 2 designates a runner section the numeral 3 designates a gate section, and the numeral 4 designates a cavity section. The gate section is designed to reduce the cross-sectional area of the flow of the plastic by 15 to 20% after the flow passes through the runner section.
FIGS. 6, 7, and 8 show cross-sectional views of lines A to A, lines B to B, and lines C to C, respectively.
The operation of the introduction of the thermo-hardening plastics into the cavity section 4 from the cull section 1 will now be described.
Thermo-hardening plastic, such as epoxy resin, is placed into a transfer and then injected into a metal mold heated up to a temperature of 150.degree. to 190.degree. C. The plastic in injected under an injecting pressure of 50 to 100 kg/cm.sup.2 and an injection speed of 5 to 10 mm/sec. The injected plastic is melted and reacted so as to harden by receiving the heat from the metal mold. In this prior art metal mold the plastic is heated up to the temperature of the mold and completely melted only at the gate section 3.
FIG. 9 shows an example of viscosity versus time characteristics of the plastic at a temperature X.degree.C., for example, 160.degree. C. In FIG. 9, the region 1 is a region where the plastic is melted before the plastic enters the completely melted state at the temperature X.degree.C., and the region 2 is a region where the hardening of the plastic is conducted.
In this prior art plastic molding metal mold of such a construction, the plastic reaches the temperature of the metal mold and enters the completely melted state at only one point along the way of the flow of the plastic, that is, at the gate section 3. Accordingly, there was a limit upon the stability of the plastic in the molded state of the product which is obtained through the uniformly melted state of the plastic, and especially, upon the reduction of voids (internal voids) existing inside the product. There is also a difficulty in reducing internal voids up to a desired amount even though the study of the condition of the molding and the flowing characteristics of the plastic.